1. Field of the Invention
The present invention relates to a digital television (TV) broadcast signal receiver for receiving digital television broadcast signals such as ATSC (Advanced Television Systems Committee).
2. Description of the Related Art
In the North American Continent where ATSC digital television (hereafter often referred to simply as “TV broadcast” or “digital TV broadcast”) is broadcast, big cities spread across the plains. Accordingly, digital TV broadcast signals, which are broadcast in big cities or their suburbs, can be received in other cities or intermediate areas between the cities. From the viewpoint of each TV user or viewer, digital TV broadcast signals come from various directions. Thus, it is necessary for the TV user to adjust the direction of an antenna in the direction of a transmission tower transmitting a TV broadcast signal of a TV program which the user wishes to watch.
For this reason, multi-directional antennas such as a so-called smart antenna are put to practical use. The EIA (Electronic Industries Association)-909 standard provides a smart antenna having sixteen equiangular receiving directions in 360 degrees. A digital TV broadcast signal receiver conforming to the EIA-909 standard is set to be able to adjust or switch an active signal receiving direction of the multi-directional antenna such as the smart antenna to every one of the sixteen receiving directions.
Some of such digital TV broadcast signal receivers have an automatic scanning function to select the receiving direction of an antenna in which a digital TV broadcast signal of a channel selected by a TV user can be best received. This can be done by scanning the sixteen directions for each channel, and by setting the best receiving direction as a receiving direction for the channel, because usually the position of an antenna or a transmission tower of a broadcast station does not change. However, in countries like the United States of America, broadcast stations may go into bankruptcy while it is relatively easy to open new broadcast stations. Accordingly, channels which could not previously be received may become receivable, while channels which could previously be received may become unreceivable. Furthermore, the receiving conditions of digital TV broadcast signals may change due to newly built high rise buildings. For this reason, it may be necessary to reset the best receiving direction for the channel selected by the TV user.
Generally, a conventional digital TV broadcast signal receiver sets, as a best receiving direction for each channel, the direction in which the received digital TV broadcast signal shows the highest signal intensity among those in the channel. Note that it is known to determine the receiving direction of an antenna using the received signal intensities, as disclosed in e.g. Japanese Laid-open Patent Publication Hei 7-7442 which describes a method of controlling an antenna to automatically track a satellite.
However, the following problems still exist. If a huge construction such as a high rise building exists near a receiving antenna, a digital TV broadcast signal transmitted from a transmission tower in a certain receiving direction of the receiving antenna and reflected by the huge construction (reflected wave) may be superimposed on the digital TV broadcast signal transmitted directly from the transmission tower so as to form a combined signal. The combined signal may have a higher intensity than a digital TV broadcast signal transmitted directly from the transmission tower in another receiving direction of the receiving antenna which, without the reflected wave, would be primarily the best receiving direction of the receiving antenna. In such case, if the receiving direction of the receiving antenna in which the received digital TV broadcast signal of a channel shows the highest signal intensity among those in the channel is simply set to be the best receiving direction of the receiving antenna for the channel, then the digital TV broadcast signal in the receiving direction, as set to be the best, may be degraded due to the influence of so-called multipath because the reflected digital TV broadcast signal is superimposed on the digital TV broadcast signal transmitted directly from the transmission tower.
Usually, a digital TV broadcast signal receiver has an error correction function, whereby errors in the digital TV broadcast signal occurring during the transmission of the digital TV broadcast signal can be corrected to some extent. However, if the digital TV broadcast signal is heavily degraded, the errors may not be corrected. In addition, the signal intensity of the received signal in a digital TV broadcast signal receiver is not decisive in determining the best receiving direction if the digital TV broadcast signal can be received accurately and without errors. Thus, the receiving direction of the receiving antenna in which the received digital TV broadcast signal of a channel shows the highest signal intensity among those in the channel is not necessarily the best receiving direction in which the digital TV broadcast signal can be prevented from being degraded and can be accurately received.